1. Field of the Invention
The present invention relates to a carrier, a developer, a developer container, an image forming method and a process cartridge.
2. Description of the Related Art
Electrophotographic image forming methods include forming an electrostatic latent image on an image bearer such as a photoconductive material, transferring a charged toner thereto to form a visible image (toner image), transferring the toner image onto a recording medium such as a paper, and fixing the toner image thereon to form a final output image. Recently, electrophotographic copiers and printers are rapidly developed from monochrome to full-color, and full-color markets are expanding.
The electrophotographic image forming methods typically include overlaying three primary color toners, i.e., yellow, magenta and cyan toners or 4 color toners including a black toner to reproduce all colors. Therefore, to produce a sharp full-color image having good color reproducibility, the surface of a fixed toner image has to be smooth to decrease light scattering. This is why many conventional full-color copiers produce images having medium to high glossiness of from 10 to 50%.
As methods of fixing a dry toner image on a recording medium, contact heat fixing methods using a heated roller or a heated belt having smooth surface are typically used. Although the methods have high heat efficiency and are capable of fixing at high speed and imparting gloss and transparency to color toners, offset problems wherein a part of a toner image adheres to a fixing member and transfers to another image occur because of separating the fixing member from the melted toner image after contacting the surface of the fixing member thereto upon application of pressure.
For the purpose of preventing the offset problems, the surface of the fixing member has typically been coated with a silicone rubber or a fluorine-containing resin, and further applied with a release oil such as a silicone oil. However, although this method is quite effectively used to prevent the offset problem, a release oil applicator is required and the resultant fixer becomes large. Therefore, a monochrome toner having high viscoelasticity when melted so as not to internally break is used in order not to apply a release oil to the fixing member (oilless) or to apply a small amount thereof. The monochrome toner has a binder resin with controlled molecular weight distribution and includes a release agent such as a wax.
In addition, even full-color image forming apparatuses are becoming oilless for the purpose of being downsized and simplified. However, as mentioned above, to improve color reproducibility of a color toner, the color toner needs to have lower viscoelasticity because the fixed color toner image is required to have smooth surface. Therefore, the color toner has offset problems more than the monochrome toner and makes it more difficult to make a fixer oilless or use a small amount of oil. A toner including a release agent has higher adherence to an image bearer and lower transferability to a transfer paper. Further, the release agent therein contaminates friction-charged members such as a carrier and lowers the chargeability thereof, resulting in deterioration of the durability of the toner.
On the other hand, hard high-strength covering layers are typically formed on carriers with suitable resins for the purpose of preventing toner constituents from film coating over the surface thereof, leveling the surface thereof, preventing oxidization thereof, preventing deterioration of moisture sensitivity thereof, extending lives of developers, preventing adherence of the carriers to the surfaces of photoreceptors, protecting photoreceptors from being damaged or abraded by the carriers, controlling charge polarity thereof and controlling charge quantity thereof. For example, Japanese Laid-Open Patent Publication No. 58-108548 discloses a carrier coated with a specific resin material; Japanese Laid-Open Patent Publications Nos. 54-155048, 57-40267, 58-108549, 59-166968, 6-202381 and Japanese Patent Publications Nos. 1-19584 and 3-628 disclose carriers including various additives in their coated layers; Japanese Laid-Open Patent Publication No. 5-273789 discloses a carrier having an additive on the surface thereof; Japanese Laid-Open Patent Publication No. 9-160304 discloses a carrier including a particulate electroconductive material having a diameter larger than the thickness of its coated layer; Japanese Laid-Open Patent Publication No. 2001-188388 discloses a carrier wherein the magnitude relation between the thickness of its coated layer and the diameter of the particulate material is specified; and Japanese Laid-Open Patent Publication No. 2005-024809 discloses a carrier wherein the particulate material is surface-treated. Japanese Laid-Open Patent Publication No. 8-6307 discloses a method of using a benzoguanamine-n-butylalcohol-formaldehyde copolymer as a main component for coating materials for carriers. Japanese Patent No. 2683624 discloses a method of using a crosslinked material of a melamine resin and an acrylic resin.
However, these conventional technologies insufficiently improve durability and prevent the carrier adherence. They still have problems of toner spent on the surface of a carrier, causing unstable charge quantity thereof, abrasion of the coated layer and exposition of a core material of the carrier, causing lower resistivity thereof. Initially, quality images can be produced, but the more images are produced, the lower the quality thereof.
In addition, Japanese Laid-Open Patent Publication No. 2001-188388 discloses a method of forming a thin coated film having concavities and convexities on a carrier, but the thin coated film does not have a sufficiently long life because of being abraded sooner and the resistivity of the carrier deteriorates. When simply making the coated film thicker, the surface thereof does not have sufficient concavities and convexities to prevent the toner spent.
Further, demands for producing images more quickly and finely are ever-increasing, and a developer receives more stress than ever and even carriers conventionally having had long lives do not have sufficient lives. Carbon black is typically used as a resistivity adjuster for carriers, and the resultant color image may be contaminated due to desorption of the carbon black.
Because of these reasons, a need exists for a carrier having good durability, capable of preventing adherence thereof.